Revamp CLI serve syntax

[chocolatkey]

This changes the current readium serve <directory> [flags] syntax, moving the directory parameter to a --file-directory flag.
It also adds a --scheme/-s flag that accepts a list of schemes to enable for accessing content (file, http, https, s3, gs). file is enabled by default. If a scheme is not enabled, but relevant flags are specified, warnings are logged to help with troubleshooting. Certain schemes also require certain relevant flags.
To help with migration, an explanatory message is displayed if a <directory> argument is still provided (no arguments expected, base directory for local files is now set with a flag).
The Docker container has been modified to no longer embed any test publications, and instead by default run service http and https schemes only.

Example commands:

readium serve --s3-endpoint https://xxx --s3-access-key yyy --s3-secret-key zzz -s s3
readium serve --file-directory ./publications
readium serve -s http,https

Closes #23

[HadrienGardeur]

Since file is used by default, is there a default directory as well?

[chocolatkey]

@HadrienGardeur There is not. If you run the command without any arguments you will get Error: file scheme is enabled, but no local directory was specified with the --file-directory flag

[HadrienGardeur]

LGTM